Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 621,787	$ 618,264	$ 627,533	$ 596,117	$ 646,365	$ 629,735	$ 619,130	$ 562,684	$ 2,463,701	$ 2,457,914	$ 2,116,737
Operating income	65,654	53,357	58,442	48,019	74,931	63,386	50,783	35,115	225,472	224,215	143,820
Net income	$ 48,496	$ 39,044	$ 43,318	$ 36,086	$ 54,563	$ 47,514	$ 38,264	$ 27,807	$ 166,944	$ 168,148	$ 202,889
Basic earnings per share	$ 0.70	$ 0.56	$ 0.62	$ 0.51	$ 0.77	$ 0.67	$ 0.53	$ 0.38	$ 2.40	$ 2.35	$ 2.81
Diluted earnings per share	$ 0.70	$ 0.56	$ 0.62	$ 0.51	$ 0.77	$ 0.66	$ 0.53	$ 0.38	$ 2.38	$ 2.33	$ 2.80